This invention relates to the preparation of novel cyclic ureas, and more particularly to N-acyl substituted cyclic ureas. These compounds are particularly useful either directly or as an intermediate to prepare a compound that can be copolymerized and used as a polymer crosslinker, more specifically functioning as a blocked isocyanate.
The formulation of one-component polyurethane forming systems which use "blocked" isocyanates are well known in the coating art. Upon heating the one-component system containing blocked isocyanates, free isocyanates are generated to react with the polyol to form the polyurethane. The need for storage in separate containers is eliminated since the system remains stable until heated.
However, the blocked isocyanates suffer from the disadvantage of volatile or fugitive blocking groups. These groups, often compounds such as phenol, alcohols and the like; are volatile at the curing temperature, and need to flash off, creating emissions problems. If the volatiles are entrained in the coating, they cause loss of strength. Bis-acyl cyclic ureas have been used as blocked isocyanates, and do not have the problem of volatile leaving groups, since the blocking group is part of the same molecule. These, however, do have a disadvantage of being difficult to disperse into the polyol, requiring such equipment as a ball mill. This mixing requirement may lead to non-homogeneous polymers.
We have now found a novel class of n-acyl substituted cyclic ureas which are particularly useful as blocked isocyanates in stable one-component polyurethane systems.
The prior art discloses a wide variety of substituted cyclic ureas which are useful for various purposes. U.S. Pat. Nos. 4,138,398, 4,217,436 and 4,410,689 disclose bis cyclic ureas which are useful as masked isocyanates and U.S. Pat. No. 4,111,877 discloses a N-cyclic ureido alkylamino derivative which is useful in imparting wet adhesion properties to water based paints. Other prior art disclosures of substituted cyclic ureas include U.S. Pat. Nos. 2,613,212, 2,881,155, 4,104,220 and 4,151,142.